1. Field of the Invention
The present invention relates to a noise control device, and particularly relates to a noise control device for actively reducing an unspecified number of noises arriving at a control point in a three-dimensional space.
2. Description of the Background Art
There is a known concept of so-called active noise control for reproducing, from a control speaker, a sound which is in an antiphase to a noise, thereby negating the noise. First, active noise control based on analogue feedback control (hereinafter, referred to as a FB control) was put to practical use. Currently, this analogue FB control is commonly used in a headphone or the like. In recent years, with the development in digital devices such as DSP and in digital signal processing technology, active noise control based on feedforward control (hereinafter, referred to as FF control) using adaptive filters, is in practical use for air-conditioning duct, refrigerator, automobile or the like. In the case of using the analogue FB control, the cost thereof can be kept relatively low. However, since it is difficult with the analogue FB control to realize complex control characteristics, a control for reducing a plurality of noises arriving at a control point in a three-dimensional space cannot be performed by the analogue FB control. On the other hand, since it is relatively easy with the FF control using adaptive filters to realize complex control characteristics, a control for reducing a plurality of noises arriving at a control point in a three-dimensional space can be performed by the FF control. Therefore, the FF control using adaptive filters is used in the case where it is desired to reduce a plurality of noises arriving at a control point in a three-dimensional space.
Briefly described below with reference to FIG. 20 is a principle of the FF control using adaptive filters. FIG. 20 shows a circuit structure which realizes a conventional FF control using adaptive filters. In FIG. 20, there exist noise sources N1 to N4 which are not correlated with each other and which are independent from each other. Performed in FIG. 20 is a control for reducing, at an error microphone 1050 which is a control point, respective noises from the noise sources N1 to N4. A noise microphone 1011 detects a noise from the noise source N1, and outputs the noise as a noise signal to an adaptive filter 1021. Similarly, a noise microphone 1012 detects a noise from the noise source N2, and outputs a noise signal to an adaptive filter 1022; a noise microphone 1013 detects a noise from the noise source N3, and outputs a noise signal to an adaptive filter 1023; and a noise microphone 1014 detects a noise from the noise source N4, and outputs a noise signal to an adaptive filter 1024.
The adaptive filter 1021 generates a control signal which is in antiphase to and has a same sound pressure as the noise arriving at the error microphone 1050 from the noise source N1. Similarly, the adaptive filter 1022 generates a control signal which is in antiphase to and has a same sound pressure as the noise arriving at the error microphone 1050 from the noise source N2; the adaptive filter 1023 generates a control signal which is in antiphase to and has a same sound pressure as the noise arriving at the error microphone 1050 from the noise source N3; and the adaptive filter 1024 generates a control signal which is in antiphase to and has a same sound pressure as the noise arriving at the error microphone 1050 from the noise source N4. The control signals generated by the adaptive filters 1021 to 1024 are combined by an adder 1030, and then reproduced by a control speaker 1040 as a control sound. At the error microphone 1050, each of the noises from the noise sources N1 to N4 interferes with the control sound from the control speaker 1040, and a difference between the control signal and the sum of the noises is detected as an error signal. The error signal is inputted to each of the adaptive filters 1021 to 1024. The adaptive filters 1021 to 1024 each update a filter coefficient thereof so as to minimize the error signal. A specific method for updating the filter coefficient is, for example, the Filtered-X LMS algorithm. By updating each filter coefficient so as to minimize the error signal, a control sound, which is in antiphase to and has a same sound pressure as each of the noises from the noise sources N1 to N4, is eventually reproduced by the control speaker 1040. As a result, each noise arriving at the error microphone 1050 which is a control point is reduced at the error microphone 1050.
Next, operations of the adaptive filters 1021 to 1024 using the Filtered-X LMS algorithm will be described in detail. It is assumed here that a transfer function from the noise source N1 to the error microphone 1050 is G1; a transfer function from the noise source N2 to the error microphone 1050 is G2; a transfer function from the noise source N3 to the error microphone 1050 is G3; and a transfer function from the noise source N4 to the control point error microphone 1050 is G4. It is also assumed here that a transfer function from the control speaker 1040 to the error microphone 1050 is C; a control transfer function of the adaptive filter 1021 is H1; a control transfer function of the adaptive filter 1022 is H2; a control transfer function of the adaptive filter 1023 is H3; and a control transfer function of the adaptive filter 1024 is H4. Note that, C is preset in the adaptive filters 1021 to 1024. Here, G1 to G4, C, H1 to H4 are transfer functions each represented by a frequency region. Further, the control transfer functions H1 to H4 are filter coefficients which are respectively updated at the adaptive filters 1021 to 1024. In order to reduce the noises at the error microphone 1050 under this condition, the filter coefficients may be updated at the adaptive filters 1021 to 1024 such that, ideally, the noises are eliminated (i.e., a level of each noise is reduced to 0) at the error microphone 1050. To be specific, the respective filter coefficients at the adaptive filters 1021 to 1024 may eventually converge to the following coefficients:H1=−G1/C H2=−G2/C H3=−G3/C H4=−G4/C 
Generally speaking, in order to reduce a plurality of noises arriving at the control point, it is required that the principle of causality is satisfied, and that noise signals detected by the noise microphones 1011 to 1014 are highly correlated to noise signals from the noise sources N1 to N4 which are to be detected at the error microphone 1050.
First, in order to satisfy the principle of causality, the following equation (1) needs to be satisfied:τn≦Tn−t  (1)Here, n is an integer no less than 1; Tn is a time which is required for a noise to arrive at the error microphone 1050 from a noise source Nn; τn is a time which is required, after the noise is generated at the noise source Nn, for the generated noise to be signal-processed by an adaptive filter 102n via a noise microphone 101n and then radiated from the control speaker 1040 as a control sound; t is a time which is required for the control sound radiated from the control speaker 1040 to arrive at the error microphone 1050. In the FF control using adaptive filters, a particular amount of time is required from when the generated noise is detected at a noise microphone 101n to when the control sound is reproduced by the control speaker 1040 (i.e., signal processing time). For this reason, in order to satisfy the equation (1), the noises are required to be detected as much as possible near the noise sources N1 to N4, respectively. By detecting the noises near the noise sources N1 to N4, time periods from when the noises are generated by the noise sources N1 to N4 to when the noises are detected by the noise microphones 1011 to 1014, can be shortened in the time periods τ1 to τ4, respectively. Then, the aforementioned signal processing time can be extended by the shortened time. Thus, detecting the noises near the noise sources N1 to N4 allows the signal processing time, which is necessary to perform the FF control using the adaptive filters, to be securely obtained.
Next, when the noises detected by the noise microphones 1011 to 1014 are not highly correlated to the noises arriving at the error microphone 1050 from the noise sources N1 to N4, the filter coefficients of the adaptive filters 1021 to 1024 do not converge. Therefore, it is necessary to increase the correlation. In order to increase the correlation, it is necessary that all the noises from the noise sources N1 to N4 are detected, and it is desired that the noises are each detected separately.
Thus, all the noises from the noise sources N1 to N4 are required to be separately detected near the noise sources N1 to N4, in order to satisfy the principle of causality and increase the correlation between the noises detected by the noise microphones 1011 to 1014 and the noises from the noise sources N1 to N4 which are to be detected at the error microphone 1050.
Next, a conventional FF control disclosed in a patent document will be described as a reference. Japanese Laid-Open Patent Publication No. 3-203792 (hereinafter, referred to as Patent Document 1) gives a description of a device capable of reducing, in a vehicle cabin, an automobile engine sound and a road noise caused by a vibration transmitted from a road surface. The engine sound and the road noise caused by the vibration transmitted from the road surface are detected as separate noises which are respectively generated by a plurality of noise sources and which are not correlated to each other. To be specific, a crank angle signal based on an engine speed is detected as a signal highly correlated to the engine sound. Also, suspension vibrations caused by road bumps, which are detected by vibration pickups provided at respective suspensions, are detected as signals highly correlated to the road noise. These signals are each separately processed by a corresponding adaptive filter, and the processed signals are each reproduced as a control sound from a speaker in the vehicle cabin. An error microphone is provided near a seat in the vehicle cabin. Each adaptive filter updates a filter coefficient thereof so as to minimize an error signal from the error microphone. As a result, the engine sound and the road noise arriving at the error microphone are reduced. Thus, Patent Document 1 also indicates a necessity to separately detect each noise when reducing noises from a plurality of noise sources.
Japanese Laid-Open Patent Publication No. 4-298792 (hereinafter, referred to as Patent Document 2) gives a description of a device capable of reducing, in a washing machine, a plurality of noises generated in the washing machine. Generally speaking, a washing machine transmits rotation of a motor to water in a washing tub by a gearbox or the like, so as to cause the water in the washing tub to swirl, thereby washing cloths or the like in the washing tub. Here, not only the motor but also the gearbox, an exterior part of a main body of the washing machine, the washing tub and other parts each vibrate and generate a noise. A noise mainly containing a vibration frequency of the motor can be detected by detecting the noise of the motor. However, there is a possibility that a non-linear component is generated when the vibration is transmitted. Accordingly, due to the vibration of the motor, a vibration of another component is excited, and this generates a noise containing a different vibration frequency from the vibration frequency of the motor. In this case, there is no correlation between the noise, which mainly contains the vibration frequency of the motor, and the noise generated by said another component, and these noises can be considered to be different from each other. Therefore, in Patent Document 2, the noise from the washing tub is detected by a vibration pickup sensor provided at the washing tub; the noise from the exterior part of the main body of the washing tub is detected by a vibration pickup sensor provided at an inner surface of the exterior part; the noise from the motor is detected by a vibration pickup sensor provided at the motor; the noise from the gearbox is detected by a vibration pickup sensor provided at the gearbox; and a muffled noise generated within the washing machine is detected by a sensor provided within the washing machine. Signals of the detected noises are signal-processed at a main control section, and reproduced from a control speaker provided within the washing machine. This reduces the noises generated from the respective noise sources. Thus, Patent Document 2 also indicates the necessity to separately detect each noise when reducing noises from a plurality of noise sources.
As described above, in order to reduce a plurality of noises by performing the conventional FF control described in FIG. 20, Patent Document 1 or Patent Document 2, there is a necessity to separately detect each noise, and therefore, a position of each noise source needs to be specified in advance. In the case of the automobile of Patent Document 1, positions of sources of the noises generated by the automobile (the engine sound and road noise) can be specified in advance. The same is true for the case of the washing machine of Patent Document 2.
However, noises caused by surroundings of such a vehicle as an automobile or a train (e.g., noises from other vehicles or echoes occurring when the vehicle enters a tunnel), change in accordance with a change in the surroundings of the vehicle, which change occurs when, e.g., the vehicle moves to a different location. For this reason, positions of sources of the noises caused by the surroundings of the vehicle cannot be specified in advance. Further, there is a case where it is desired, when, e.g., you have a relaxed time on a sofa at home, to reduce noises from outside as well as noises caused by room appliances (i.e., daily life noises such as vacuum cleaner noises, television sounds which you hear when other family members are watching TV, kitchen sink noises, ventilator noises, or the like). In such a case, the daily life noises are not always the same, and the daily life noises of each home (family) are different. Therefore, it is necessary to consider that the daily life noises arriving at the sofa always change, and for this reason, it is of course impossible to specify positions of noise sources in advance. The same is true for a case where it is desired, at a workspace such as a factory or office, to reduce noises coming from surroundings of the workspace, and thereby allow a person therein to concentrate on his/her work.
Thus, it can be considered that a control point in a three-dimensional free space such as a vehicle cabin, house, factory or an office, is always surrounded by an unspecified number of noise sources. In other words, it can be considered that an unspecified number of noises arrive at the control point. Since the conventional FF control shown in FIG. 20, Patent Document 1 or Patent Document 2 is unable to specify positions of sources of such an unspecified number of noises, it is almost impossible for the conventional FF control to reduce the unspecified number of noises at the control point.
Further, in a space surrounded by a floor, ceiling and walls, a noise generated by a noise source reflects on the floor, ceiling and walls. Such a reflected diffuse noise becomes a different noise from the noise generated by the noise source, and this further increases difficulty in specifying a position or a direction of the noise source. Therefore, it is even more difficult for the conventional FF control shown in FIG. 20, Patent Document 1 or Patent Document 2 to reduce, at the control point, the unspecified number of noises in a space surrounded by a floor, ceiling and walls.
Hereinafter, the reason for the conventional FF control being unable to reduce an unspecified number of noises, will be described in further detail. FIG. 21 shows a noise control device for performing noise control for an area near the head of a listener A in a laboratory B in which sound reflectivity is relatively high. In FIG. 21, noises which are not correlated to each other and which are independent from each other are respectively outputted from four noise speakers 1001 to 1004 which are provided relatively near the listener A. The laboratory B has a size of 8.5 m×8.5 m×2.5 m. The listener A is distant by approximately 1 m from each of the noise speakers 1001 to 1004. Noise microphones 1011 to 1014 are provided right in front of the noise speakers 1001 to 1004, respectively. The noise microphones 1011 to 1014 each detect a different noise. Each detected noise is inputted to a multi-channel adaptive filter 1020. The multi-channel adaptive filter 1020 is the same as adaptive filters 22A1 to 22D1, 22A2 to 22D2, 22A3 to 22D3, 22A4 to 22D4 and 22A5 to 22D5 shown in FIG. 3 of Patent Document 1. To be specific, the multi-channel adaptive filter 1020 has: four adaptive filters for the noise microphone 1011, which have different transfer functions (filter coefficients) from each other; four adaptive filters for the noise microphone 1012, which have different filter coefficients from each other; four adaptive filters for the noise microphone 1013, which have different filter coefficients from each other; and four adaptive filters for the noise microphone 1014, which have different filter coefficients from each other. The four filter coefficients set for each noise microphone respectively correspond to control speakers 1041 to 1044. A signal outputted from the multi-channel adaptive filter 1020 to the control speaker 1041 is a sum of signals respectively processed by adaptive filters each having a filter coefficient corresponding to the control speaker 1041. Similarly, a signal outputted to the control speaker 1042 is a sum of signals respectively processed by adaptive filters each having a filter coefficient corresponding to the control speaker 1042; a signal outputted to the control speaker 1043 is a sum of signals respectively processed by adaptive filters each having a filter coefficient corresponding to the control speaker 1043; and a signal outputted to the control speaker 1044 is a sum of signals respectively processed by adaptive filters each having a filter coefficient corresponding to the control speaker 1044. Note that, it is assumed that the experiment in FIG. 21 satisfies the aforementioned relational equation (1) of the principle of causality.
Error microphones 1051 to 1054 are provided near the ears of the listener A. Near the ears of the listener A, noises from the noise speakers 1001 to 1004 interfere with control sounds from the control speakers 1041 to 1044. Differences between the noises and the control sounds are detected as error signals at the error microphones 1051 to 1054, respectively. The detected error signals are inputted to the multi-channel adaptive filter 1020. The multi-channel adaptive filter 1020 updates a filter coefficient of each adaptive filter so as to minimize the error signals in total. As a result, the noises from the noise speakers 1001 to 1004 are respectively reduced at the error microphones 1051 to 1054 provided near the ears of the listener A. A specific method for updating the filter coefficient of each adaptive filter is, e.g., the MEFX (Multiple Error Filtered-X) LMS algorithm.
FIG. 22 shows coherence (degree of correlation) between a noise signal inputted to the noise speaker 1001 and a noise signal detected by the noise microphone 1011 in the experiment of FIG. 21. As shown in FIG. 22, noise signals inputted to the noise speakers 1001 to 1004 are each a pink noise which is band-limited between 100 Hz and 1000 Hz. It is understood from FIG. 22 that the correlation between the noise signals is high in the same band (100 Hz to 1000 Hz) as that of the noise signal inputted to the noise speaker 1001. The reason for this is that the noise microphone 1011 is provided right in front of the noise speaker 1001. For the experiment shown in FIG. 21, a noise reduction effect obtained at the error microphone 1051 is shown in FIG. 23, and a noise reduction effect obtained at the error microphone 1052 is shown in FIG. 24. As is clear from FIGS. 23 and 24, the noise reduction effects are each obtained by 10 dB or more in the band from 100 Hz to 1000 Hz.
Next, consider a case, as shown in FIG. 25, where the noise speakers 1001 to 1004 are provided so as to be more distant from the listener A as compared to the case shown in FIG. 21. In FIG. 25, the listener A is distant by approximately 2 m from each of the noise speakers 1001 to 1004. Since the other conditions herein are the same as those in FIG. 21, FIG. 25 does not show the multi-channel adaptive filter 1020 and wirings thereof. Also, it is assumed that the experiment in FIG. 25 satisfies the aforementioned relational equation (1) of the principle of causality.
FIG. 26 shows coherence between a noise signal inputted to the noise speaker 1001 and a noise signal detected by the noise microphone 1011 in the experiment of FIG. 25. As shown in FIG. 26, a correlation between the noise signals is lower than in the case of FIG. 22. The reason for this is that the noise microphone 1011 is provided in a distant position from the noise speaker 1001. When the noise microphone 1011 is provided in such a distant position, the noise microphone 1011 detects not only the noise from the noise speaker 1001 but also a noise having complexly reflected on a floor, ceiling and walls as well as other noises from the noise speakers 1002 to 1004. For this reason, the correlation between the noise signals is lower in FIG. 26 than in FIG. 22. Further, it can be expected that the noise having complexly reflected on the floor, ceiling and walls reaches the listener A without being detected by any of the noise microphones 1011 to 1014. This means that a new sound source has appeared near the listener A, and therefore, a position of this sound source cannot be specified in advance. As a result, there are unspecified number of sound sources surrounding the listener A. Consequently, as shown in FIGS. 27 to 30, a noise reduction effect cannot be obtained. With respect to the experiment in FIG. 25: FIG. 27 shows a noise reduction effect at the error microphone 1051; FIG. 28 shows a noise reduction effect at the error microphone 1052; FIG. 29 shows a noise reduction effect at the error microphone 1053; and FIG. 30 shows a noise reduction effect at the error microphone 1054.
As described above, a noise reduction effect cannot be obtained by the conventional FF control using adaptive filters, unless positions of noise sources are specified in advance, and noises from the specified noise sources are each separately and entirely detected. Accordingly, when there are an unspecified number of noise sources surrounding the listener A, noises therefrom cannot be reduced by the conventional FF control. This is suggested by Patent documents 1 and 2 since Patent Documents 1 and 2 do not give a specific description about controlling noises from an unspecified number of noise sources.